1. Field of the Invention
The invention relates to the field of mask alignment techniques for integrated circuit fabrication.
2. Prior Art
In recent years, the advances in integrated circuit lithography have enabled the fabrication of much higher density circuits. The masking steps used today for these circuits bear little resemblance to the contact masking technique commonly used ten years ago.
Currently, one of the more commonly employed masking techniques is the "projection system." An integrated circuit master pattern is projected onto the wafer and stepped from location-to-location until the entire wafer is exposed. In some cases, the integrated circuit master pattern contains several, for example, four identical patterns, thus four circuits are exposed at each step. These projection systems vary greatly, and in some instances, a one-to-one ratio between the mask pattern and projected image is used; in other cases, the ratio is as high as 10 to 1. For a discussion of direct-step-on-the-wafer masking technique, see "Wafer Steeper Steps Up Yield and Resolution in IC Lithography," Electronics, Aug. 16, 1979, beginning at page 109.
When the integrated circuit master pattern of each mask is stepped on the wafer, it must be aligned at each location. This, of course, is necessary to assure proper alignment of the circuit elements in each of the layers. Prior art markers are often fabricated during the initial "front end" steps, and subsequent masks are aligned to these markers. Frequently, these markers consist of openings in a field-oxide layer.
During the alignment, the wafer is covered with a photoresist (generally of uneven thickness). The markers are sighted visually through the photoresist layer and the projected image from the mask is then aligned to the markers. Once this is done, the photoresist can be exposed through the mask, to "transfer" the mask pattern to the photoresist.
The manual alignment systems which use multi-wavelength light work fairly well with prior art markers. The multi-wavelength light is not extinguished by interference patterns which occur in the photoresist layer, making it easier to align to the markers. A problem occurs, however, where a single wavelength of light (or a narrow band of wavelengths) are used. Then, extinction reduces the contrast, making it difficult to align to the markers. This is particularly a problem with automatic alignment systems.
As will be seen, the present invention provides a new marker which is particularly useful with automatic alignment, projection systems using a single wavelength or narrow band of light for alignment (e.g., green or yellow).